Volume control circuits



ug. 15, 1933. N C- NORMAN v 1,922,602

VOLUME CONTROL CIRCUITS Filed Jan. 27, 1931 LPF /NVENTOR N. C. NORMAN ATTORNEY Patented Aug. 15,r 19433 isaactz votos/rr. contract. cntctil:rs`

Natl="riel C. Norman, New ark, N. Y., assigner ll Telephone Laboratories, liz-acorncrateri.,

New York', N. Y.; a Corporation of New York rtsi. serial Nspsnsoi `9 claims. (ci. 17e-44) This invention relates to signal wavetransmission systems and particularly to means for con* trolling the volume range of the signals transjmitted thereover. v f L An object of the, invention is to improve the operation of means for automatically Vcontrolt ling transmission in a signal transmission system.

In the transmission of signals representing speech or music` havingf a wide variation'in vol- 10 urne, such as is obtained, for example, `in connection with the broadcasting of orchestra and public address programs, over telephone lines 'or radio telephone systems, it is desirable to maintain the power level of the transmitted signals within definite limits. An upper limit Vfor the power leveloi the program tov be transmitted is necessaryto vprevent overloading oi line apparatus, such as vacuum tube repeaters, ,and to reduce objectionable' cross-talla, while a lower limit is necessary so that ythe noiseinherent in the apparatus used or' inthe line will not be troublesome.

The U. S. Patent 1,757,729, issued May 6, 1930 to R. C. Mathes discloses a system for effectively transmitting signals havinga wide range of vol at the input end Within the transmission Alimits of the system by causing the loss introduced4 therea't to increase as the input level increases, and to correspondingly expand the volume range of the received signals at, the output end by a converse "operation thereat. According' to the speciiic method disclosed by Mathes, a small por '40 tion of the signal energy is divertedfrom the* ity forthe overall control circuit becomes sorne function of Ythe input intensity. At the input Aof the system.

the control circuit is arranged so that the recti- The rectified current, which Will have .ned vcurrent applies positive voltages onthe grids of the shunt tubes causingthe value of the shunt l impedance lin the main signaling channel at the transmitting station to decrease Withincreased input and .thus to compress the volume rangeof the signals" transmitted to the otherv end ofthe system; At the output of the system the control circuit isV arranged sol that the rectied'current applies negative lvoltages on the grids of the vshunt tubes causing theV value ofy thefshunt impedance in" the receiving` signal channel 4to `increase with increasedl input andthus to expand the Volume Y range of the received signals.

vThat/lathes patentfalsoydisclosesithefuse'of a f low pass filter in the control circuit at each end 70 of the system to filter out the speech'frequency components so that only the envelope of the rectified currents actuates the shunting device.

The circuits of the present invention'are inthe nature of improvements overthe volume control 75 circuits of the Mathes patent described above, the improvements mainly residing in the modification of these circuits to produce alinear relation be-Y tween the rangey of signal volumes at the input to the compressor and the range of signal volumes at' any point in ythe system betweenfthe' compressor and expander 'regardless of whether the ignal intensity at the'input to thecom'pressor is high or low, or whetherythe intensity range being transmitted is large or small.V y

Inview of the desirability of restoring at the output of ,the system the electrical energy representing the speech or music to its original volume range at the input by means of a device similar in construction to the cornpressor'atthe input 90 of the system, a lavf of compression in accordance with Equation (l) below was selected.

Where X is theinputfin transmission units above an arbitrary Zero level (which in the circuit of Vthe 'inventlonwas about 0.09 volt), Y is 'the .out-

put -in the same transmissionunits referred to the sameV Zero level, and Kr is the loss inthe saine transmission units in the compressor when X is'atthe arbitrary zero level.' The arbitrary 4*Zero is the point at lwhich the compressor begins to func-VV tion in accordance withEqu'ation (l), which' holds only when X is positive. a

The root mean square output fvoltage; the 1.10 *Y root mean'square input voltage, Ei, and the voltage, Ez, at which the compressor begins to func tion in accordance with equation (1), are related by the equation:

' Eoeaamaw 2 l for all values of Ei equal to or greater than Ez which' is 0.09 volt. If Equation (l) is written in the form:

new (EZ) +K1 it is seen that Equation (2) is identical Vwith Equation (1) provided Y 20 log =V20 log A compressor having the characteristics given by Equations (l) and (2) is obtained in accordance with the invention by inserting in front of the` rectifier in the control branch Vof a com- 'presson' such as disclosedin the Mathes patent, a

vacuum tube device which will produce in its output, voltagesv proportional at any instant. to the square root of the voltages impressed upon its input. The net result of incorporatingv this device in the control circuit ofthe compressor is to produce a linear relation between the input and outM 'put energy lin transmission Aunits for the come presser. Thisinsures that any eiect on the output of the system caused by aV change in the line Yequivalent Will remain'the same for all values of signal intensity at the input.

The loss 1L in aline having terminating impedances Z 'and afvariable shunt impedance ZS is given by the equation L (in transmission unts IOg JZ (4) which, if Written in form showsthat if Variation of ZS is to cause an appreciable variation in loss in the line, Z0 must be larger than the highest value of Z5. In vthe compressor circuit of the invention, therefore, the portion of the main signaling channel ,across which the plate circuits of the control tubes are l bridged, is designed to have a higher impedance" thanthe maximum impedance of the plate circuits of the control tubes, so that variations in the plate impedancesof the tubes will produce a wide variation in the loss introduced into the signaling channel. v

To restore the electrical energyrepresenting speech or music to its original range ci intensity variation after it has been compressed in accord-V ancewith `Equations (l) and (2), it must be sent through a device having a law of expansion in accordance with the following equation:

where X is the input of the expander in decibels above the same arbitrary zero level (about 0.09

volt) Yis theoutput of the expander in decibels;

`and K2 is the loss in decibels in the expander when X is at the arbitrary zerolevel. The arbitrary zero level is the input at which the device Ybegins to function as an expander, and for any lower in put, it is assumed that it functions as an ordinary put Vby the equation repeater and that the output is related tothe inthe output in volts.

lsignaling channel as in the compressor.

, Y=X+K2 (6) Where X is the input in transmission units below the arbitrary zero level, and Y isthe correspond-- kThe root mean square voltage relation correspondingto'Equatio'n (6) is given by the equation where Ei the input in Volts below EZ and E0 is Kz is related to lcz by the An expander obeying the lawof expansion exi pressed by Equations (5) and (7) above is obtainedl in accordance. with .the invention by a circuit. `similar to the compressor circuitof the invention vdescribed above except that the vacuum tube rooter device is eliminated in the former, the relation between applied voltages to the grids of the control tubes and the input voltages tothe control circuit-of the expander being necessarily linear because of. the previous action by the compressor, andr that the plate impedances of theV control tubes in the expander are connected in series with inste'ad'of in shunt` withY the main The series connection requires that the portion of the main signaling channel, in which the plate im-l l pedances of-the control tubes are connected in series', be designed-tc have a Vlow impedance in comparison Withthe'minimumvalue of the plate impedance of the control tubes in orderfthat a wide variation in loss in the main signal channel may be obtained with variation in the plate im- 125 l pedances of the control tubes in the expander.

The Various features and advantages of the cir;

cuits of the invention will be better understood from the following detailed description thereof when read in connection with the accompanying drawing in which:

Fig. l illustrates diagrammatically a system for.

broadcasting electrical ,energyy representing speech and music, and embodying the volume control circuits of the invention; and

vFig. 2 illustrates diagraminatically amodification ofa portion'of the circuit ofV Fig. l.

The transmission system of Fig. lcomprises a sending station A and a receiving station B connected by aV transmission mediurnML oflimited volume range capacitmsuch as a telephone cable.

IThe circuits of the sendingstation A comprise an auxiliary control circuit VCi `and a main signal transmission circuit TC. The main circuit TC includes the one-Way amplifying device 1 having..

its output connected to the input of the transmission' medium ML, and its input connected to theA circuit 2 leading to the source oi signals to be broadcast, through a section of line 3 of high im# pedance, ywhich may be a-,resistance network as indicated, and transformers e and 5.

The auxiliary control'ci'rcuit VCi hasit's'input to-bei equivalent to a"tvvoelectrode ytube,-vvhich `connected across the incoming'circuit ,2A and' the' vinput of the main signal transmission circuit TC so as to divert a portion of the incoming signal energy from the latter. "In thc circuit VCi the 'input of 'the one-"Way vamplifying devicelV is con- -nected to the circuit 2 across the potentiometer A6, and the output or the amplifying device'7 feeds into the input of the vacuum tube router device ,9 through the transformers. The'output of the Vacuum tube rooter device 9, which comprises `tlfie-'vacuunfi tubes 10 and 11, is connected to' the input of the one-Wayaainplifying device 12, the

output of which is fed through the transformer 13 to' the input of :the linear rectifyingrdevice i4.

.'The low pass lter 15 is connected between the output of the rectifyingwdeviceli and the input of the variable impedanceY element -15 comprising the threeeelec'trode space discharge devices 17 and 18, Vthe grid-lainent circuits of which Ahave a portion in common" so as to be fed in `parallel from theloutput oi nlterii, and the: plate-lainent circuits of which are connected in'push-pull relation.

"The retardation coils 19 and 20 in the respective plate-filament circuitsof tubes 17 and 18, are

bridged in series across the high impedance por.

tion 3 of the main signal transmission-circuit TC. At the receiving station B, thecutput of the transmission medium ML is associated hy the re` peating coil 21 With the input of thev one-way'. amplifying device 22 in the inain receiving signaling path RC, and alsoacross the potentiometer 22 with the input ofthe one-Way amplifying device 24 in the auxiliary control circuit VCz.

`The output ofthe one-way amplifying device 22 in the circuit RC through transformer 25, the sectionof line 27, and transformer 2G is connected to the input of the one-Way` amplifying device 28, the output 'of which is connected by the output transformer 29 to the outgoing circuitV 30.

The output'of the one-way anipliying device 24 in the control circuitVC2 feeds into thev input of a linear vacuum tube rectifying device 31,' the output of which isconnected througnthe low pass filter 32 symmetricallyrto the grid-iilaroent circuits of the two three-electrode space discharge devices 34 and Siforrning the varia'ole'irnpedance device 33. The..p1atefilament circuits or the two three-electrode-space discharge devices 34 and 35 are connected in push-pull relation, and, through the coil 36, equal portions ci which are contained iithe two platalament circuits, are connectedV voltages proportional to the square root of thev alternating current voltages impressed upon its input may be utilized for the devicel 9 in the control circuit VCi ati the sending station vA; TheV device f'illustrated is sirniiar to that employed inthe. main signaling channel in thevOlume control circuits illustratedand` de scribed in the U.' S. Patentrllo. 1,737,830, issued December 3, 1929 to G. Crisson, lit comprises tvfo three-electrodejvacuurn tubes 10 and 11, each` having its Aplate'and grid strapped together so vas are connected in Ii'aarallelvvith each other across f the input of the amplifying device 12 but inseries Withjthe secondary lWinding of the inputftransformer. l The connection issuch that the `input currents of one polarity pass througlithetubet `10 and thefinput'currents of opposite-polarity, throughthe tube 11. The input,terininals'of-'the Vamplifier 12' are connecteddirectly, across-fthe points X and Y in this circuit.

The device'9 in the control circuit VC; operates former 8 increases Withiiicreasing signal'current input to the compressor, the impedancelb'tvve'en n the cathode and anode of the two-eleinent-tubes `V10 and ll decreases, causing the loss in trans-v mission units between 4theout'put of a'rnpliiier 7` and the input to amplifier 12 to increase; When "the voltage rat the output cf transformer 8 de- "parts of the router,` it is possihleto causethis loss to vary -withsignalicurrent 'input to the compressor in such-alway' that the'voltage'a's Vsquareroot 'of thevoltage applied to thefrooter The device 14 in `control circuits VC1 at the fas followszA-s thelvoltage at the output oi' trans- `measured rat the outputof ampliiierlZ Nis Vthe sending station A. and the corresponding device '31 in the control circuit VC2`at the receiving station B'mayfbe. any suitable device for producing a linear rec-'tiiicfationioi the voltages impressed vupon itsinput, for example, aY threeelectrode 'vacuuni tube having its grid and plate strapped Vtogethertoproduce an equivalent two-element at the sending station A, and the 'low passfilterV 32 in the control circuit VS2 lat the receiving stationB, v,are 'providedffor the' purpose of effectivelyV suppressing 'froni'the rectied currents transmitted therethrough anyvoice Vfrequency componente which might resultin distortion being produced inthe main signal transmission circuit if applied to the grids of the tubes in the variable ,impedance devices 16 and'33,'respectively. nach loifv pass inten-*preferably isdesigned to have a cut-off at about to cycles per second. 4VThese'iilters nayfhedesignedin accordance with the principles s'etrforth'" in the U. S. patents to Campbell, Nos. 1,227,113 and 1.3221114, issued May 22, i917." The operation'ofthe system of Fig. 1 will nowv be described. *1 -hueV ain portion' ofthe electrical Y energyY corresponding to the speech for :rnusicl program to be broadcast; received'over the circuit 2 is' iinpressedhythe transformer .e upon the rnain vsignaltrans rniss'ion circuit TC andvis' transinitte'd throughv the' impedance portion 3- thereof and by the input transformer 5 upon the input circuit of the 'amplifier-1.

A portion of the electrical energy received over theY circuit 2 is diverted into the input of the control circuit VCL and irnpressedby the potenfv tionieter 6 upon the input circuit' ofthe amplifyl v proportional at any instant to the square root of Vthe impressed voltages. These Vsquare root voltages will beA amplified in the amplifying .device 12 Vand impressed by the transformer 13 upon the 'linear rectifying device 14. The resultingrectied energy inthe output of the device 1K4 will then I be transmitted through the low pass filter 15 which will eifectively. suppress therefrom'the frequency components Whichare above its vcut-oil?.

frequency L .The rectied variations in the output .of the lter 15',1which, because of the previous action ofk the rooter device 9, will vary in direct propor'- vtion tothe square root of the input voltages impressed `on the input of the lcontrol circuit V01 from the incoming circuit 2, are then supplied'to the input circuitfof the variable impedance device 16 comprising the three-electrode vacuumtubes 17 and 18.- The rectified variations are applied to the grid-filament circuits of tubes 17 and 18 in such manner that like rectified control potentials are applied to the grids of the two tubes symmetrically. These rectified control potentials cause likevariations in the grid voltagesof each tube and consequently equal variations in'the plate impedances of the two tubes. lAs the output circuit Vof the variable impedance device 16 is coupledby means of the series retard coils 19 and 20 directly across theportion 3 ofthe main signaling channel TC, a loss corresponding to the sum of the plate impedances of the tubes 17 and 18 in series is thuseffectively inserted in shunt tol the signaling path TC.y The'push-pull arrangement of the output circuits ,of the tubes 17 and 181 provides that, while their irnpedances are respon-f sive to changes in the .control potential applied to their grids, their combined impedanceas seen from the main transmission path TC is substantially independent of the magnitude of energy passing over that path. 1,

The poling of the grids-of thetubes 17 and l 18f With-respect to that ofthe rectier14 is impedances-of .the tubes 1-7 and 13. of;V the output impedance of device 16 varies in versely with the voltage `applied. to the grids` selected so that the values of the output irnpedances of these tubes will decreasewith increase inthe amplitude of the signals at the input to the signaling channelTC, andgftherefore, the loss inserted `in the signaling channel TC will .increase with increase in the input level and decrease with decrease in input level. As the portion 3 of the main signaling circuit TC across which the .plate impedances of thetubes` in the variable impedance devicel are ccnnected is designed to have lan impedancewhich is large inv comparison. with allvalues of .the impedance of the output of the device 16, a widevariation in the loss inserted inthe Y voice channelis obtained with variation in the plate The value of tubes 1'? and 18, which voltaga'due to the action of -the rooter device;9, is vsubstantially proportional to the square root of the input voltage to the control circuit VCi (the compressor). Due to the well known grid-voltage plate im'- pedance characteristic of a three-electrode vac-'- uum tube,` it is apparent then. thatV the value of the output impedance of the device 16 will be a function of the input voltage to the control circuit VC1,.hence the compressor characteristic may be made such that. the volume rangein. decibels'of the Vsignals delivered tothe input of theamplier 1 in path TC, ishalf the volumeA rangev in decibels of the signals received from the circuit 2. It will be evident that by propery choice of the values of the elements iny the variable impedance device 16 and those in the portion 3 of the signaling pathv TC the loss varying with input level in the signaling path TC maybe made such as to compress the volume range ofthe variations impressed by transformer 4 upon any portion 3 of that channel` Within any range capablev ofv being transmitted satisfactorily by the transmission system from that vpoint to the receiving station B. The com-. l pressed signals will be impressed by the. trans' former 5 upon the input ofthe amplifying device 1 and ampliiedv thereby to a desired average level depending upon the length vand the transmission characteristics of'the line ML. The rez sulting amplified energy is transmitted overthe .transmission medium ML to the receivingsta'- tion B where it will be impressedV by the .transformer 21upon the input circuit of theamplier 22 in the main receiving circuit RC,- and" upon the input circuit ofthe amplifying device 24 in the control circuit `VC2v across the potentiometer 23. w n

The .amplified signalsin ithe voutput ofthe amplifying device 22 in -channel RC are supplied through the transformer 25 to the low impedance section 3 of the receiving circuit RC.

l\/ieanwhile,` in the control circuit VCzI the diverted input energy is amplied. by the amplier 24 and then rectified by the 'linear rectifying device 3l. The rectied currents'in the output of the rectifyingdevice 31 pass through the low pass filter 32 whicheffectively suppressesv the frequency components whichare abovev its cutd oi' frequency.y The rectified voltages in the output circuit of the lter 32; are then supplied symmetrically to the gridlament circuits of tubes 34 and 35 in the variable impedance device 33, and, in themanner described above in connection with .the similar device 16 'in' the control circuit VC1 at the sending station A, will vary the output impedance of the two tubes 34 and 35, and, therefore, the loss inserted in the signal receivingcircuit-RC through the output coil 36 in accordance with the power level of the signals at theinput of the circuit RC. 'The poling of the grids of the tubes 34 and 35 with respect to that of the rectiiier 31 is made such that the output impedance of the tubes 34 and 35 inseries in the portion 27 of the receiving circuit RC will decrease with increase inthe amplitude of the signals impressed upon thercontrol cH'- `receiving circuit RC by tubes 34 and 35 ofthe variableirnpedance device 33, due to the operation of the compressor at station A, will be such a function ofthe voltagev impressed. on thercontrol circuit VCz thatno rootery device is ref quired therein to make the' expander (control circuit VC2) restore the signals `tol the original volume range which they hadat the input of.v the system. f i g l v.-

The expanded signals from the line. sectionf'27 of the receiving circuit RC areimpressed by lof transformer- 26 upon ,the input circuitv of theV amplifying device 28 in which they are amplified to any desired average level.V The amplified sig, naling energy in the output of `the. amplifying device 28 is impressed by the transformer 29 upon the outgoing circuits 30. i

In the compressor portion of the system of Fig. 1 only two ungrounded filament batteries are employed. This is not particularly objectionable;-

from an operation and maintenance standpoint inasmuch asthese two batteries arev filamentV heatingbatteries for thertube in the rooter device 9 and forthe linear rectifier tube 14 in the control circuit VC1 at the sending station A, and which may be dry cells. of ungroundedilament batteries may be reduced to one as indicated in the circuit of Fig. 2 by employing in place of the ordinary input transformer 8 for the rooter device 9 a special input transformer 37 having a primary Winding 38and which has been constructed,in order to makeV the input to the control circuit of the expander rise to as high a value as the maximum input to the` compressor control circuit, whenthe expanfV der is operated by the output-ofthe compressor, .it was found Hnecessary to amplify the compressoroutput about 24 decibels since the loss in the compressor was about 24 decibels at maximum input. If speech energy whose peak intensities just reach 40 decibels above the arbitrary zero level (0.09 volt) is sent` into the compressor vand then through a 24 decibel amplifier, the peak intensities at the output of the 24 decibel amplifier will be thesame as in thespeech energy before compression, but the low intensities of the speech Vwill be'brought up with respect to the highintensities in accordance withEquations (l) vand (2) That is, the difference between the strong sounds of speech and theweak soundsfwill be only half as great, in decibels, asbefore compression, and asv a result of the amplification, the weaksounds will be twice as loud with respect to the strong sounds, in decibels, as without compression. The strong ,sounds are, however, no stronger-than they'were before compression and therefore-will not overloadfany repeater ina line over which the speech might be sent, unless they" would be overloaded by. the original speech'anyway.

i But to a listener at the other end of a line .over which speech is' sent, the speech sounds appearmuchlouder when compressed and amplified asdescribed `above before going over the linel than the same speech coming over the line without compression. Since the weak sounds of speech are also' the important sounds for articulation,vit would be' expected that the use of the: compressor Valone might improve the articulation of a telephone line. Actual tests indicate that considerable improvement in-this respect is obtained. 4

Let it be supposed that there is a definite noise level on 'a telephone line at J30 decibels below the overloading point of the line repeaters, and

speech varying `over a -range of 40 decibelsis to be sent over'this line. rThe weakest sounds of the speech will then be 1G decibels below. the noise However, the numberV level if the .strongest sounds are justat the over- 'y loading point of they repeaters. But if the speech i is compressed bya .circuit such as shown atsta-A tion AA in the system of Figli, and is thenampline'd Eidecibels by amplifier l beforegoinglover the line, the weakest' soundswill belO decibels higher than the noise level, and so will ,no longer be masked by the noise. VSio' the compressorpof the invention alone maybe used to reduce interi ference from noise on teiephone'lines, Itmayf be used in a similar manner in connection with radio telephone circuit to reduce interference from static; in .connection withia circuitfor making phonograph records to reduce the surface noise; in connection with talking motion picturereproducing circuits for reducing film noise; and, in fact it may be used` in any. circuitV Where inter-` ference with speech orrnusc comes from a source such that the compressor can be inserted betweenv the point wherethe speech or music is changed from sound wavesglightwa'ves, or any other form I of energy, into electriccurrents,` and the .point where the interfering currentsenterthe circuit. .Recent tests havelbrought out yan important field of usefulness'for the expander alone in connection with ya long cable circuit, such as betweenl New York andn Chicago. .Y It was found-that one of the e'ects of the 3600 mile cable was to introduce ahigher `loss to'lhigh speech levels than to low speechlevels, so that the cable acted fas a with the-frequency: When theloss throughthe cablecircuitwas plotted as a function'of-,thexy speech intensity, the shape. of the curve indicated that Vif theexpander` vdescribedfinV this specifica-110 tioncould be made to function v according tov Equations (Bland 9) over thefirst i0 decibels vpartiall.compressor, the effect varying slightlyfV as the input increases and in accordance.withY `l Equations (5,) and -('7)`A as itheinput. furtherl increases-the compressing action of the cable ,forA strong speech currents would bey almost fully cor- Y rected., The, same method might be ,appliedrto' g 'diiferent lengths of cable te'correct other degrees of comp, ressiorr The regulation oftheinput `of the corrective expanderfwouldin allcases-be des termined byithecompressing characteristic of ther: cable; f V

'As stated above, wherehigh j sion is desired making it desirablefto restore the electrical energy representing speech -or music toithe original volumerangejthe expanderwould i be used iin addition to the compressor..l Theexpander described, howevenvaries the volume'of theinterfering noiseaswell as that'of the energy coirespondingto the VspeechV or music. -If vtheY noiselevel on. a telephone line, forfexample, werel at the arbitrary'zero inputlevel oftheexpander,

that is, `at the level at which the expander begins to-f'unction according to Eqi-iationsV V(5)v an'dtl) it er for maximuminput)i below the arbitrary zero levelas heard by a listener at the output of the expandergwhen maximum speech is coming over? the line. But, if `the'speech.now comes over the level, the'loss in-theue'xpander increases from 6` to 26 decibels andthe noise ,according-ly. drops 20 decibels as heardin the output' of the expander.` The noise, then, as heardby a listener attlrieoutf` put of the expander, varies Withrthe speech enf velope and vis always less than the noise received when speech inthe samelevel is received by direct transmission except' When-therspeech intensity isV maximum. i

would bezaboutl decibels (the loss in the expand- 1351 Y.

. las'. lin'eand' intotheexpander attthe arbitraryV Zero Y `time must be insensitive enough so that they will i not be operated by the noise, static, or other stray in the medium supplied with'saidsignals, a three-elec` -sion and expansion may be obtained.

currents. When the volumerange ofthe speech ishalved, as by compression in' a circuit such as shown' at the sending station A in Fig. 1, and the Weak sounds are then liftedabovethe noise level l" by amplicatiomthe vsensitivity of the voice operated devices can be correspondingly decreased Without clippingthe`speech. This, of course, applies only to voicejoperated devices connected enters. i n

Only onestageof compression and one stage of expansion have been shown in the system of Fig.y r1. However, it is to beunderstood that in accordance With the invention, .this system is not limited to onestage of `compression and one stage ofv expansion, but by addingjadditional apparatus similar to that shcWn several stages of compresdescribedy of the invention should `be considered las typical and not as limiting the invention, the scope of n cies Within the speech and music frequency range;

which i'sdened in the appended claims.

What'isclairned is:` l Y e 'f l. In a signal transmission system, a source trode space discharge device havingagrid cirfcuit and a plate-iilament circuit, said plate-fila- `4ment circuit being'connected effectivelyin shunt @with Saidmediumso as to yinsert an electrical loss thereinthe value of which Ais dependent on the'value of the impedance of said plate-ilament Ythe point of connection of said plate-lament circircuit; and which determines the volume range .0 ,1. of the signals transmitted'over said medium, and

means to produce'alinear relation between the range ofsignalivolumes in said medium beyond cuit thereto and the range of signal volumes at Ytheinput vof Vsaid medium, said means-,compris` -`ing' means for' impressing on the grid circuit of said device rectied voltages Whichv are'substantially proportional to the square rootof the alternatingvcurrent signal. voltagesat the input to o. .said medium to control the value of the imped- -...ceive aportion of the signalsimpressedthereon,

"said'control circuitcomprising a linear rectifier and a vdevice which Willproduce in its output voltages substantially proportional .tothesquare root of voltages impressed on its input. Y

3. vIna signal transmission system, a 'sourceof alternating current signals :comprising a plurality` of frequency` components, atransmission me-l dium -suppliedwith said signals, al three-electrode space discharge device comprising `a grid -circuit and a plate-lament circuit, said plate- -filainent; circuit being soA connected With said ltrol circuit connected to the input of said medium so as to receive aportion of the signals irnpressed thereon, and comprisinglinear signal rectifying means, means-for impressing the sig-L nal Yvoltages rrectiedthereby on said grid circuit to control the value of the impedance of the circuitsbeyondthe point where the noise circuit `such a functionv of' the signal voltages` received by said control circuit as to make the range of signal volumes expressed-in decibels in i said medium beyond the point of connection of said device thereto substantially half therange of signal volumes expressed in decibels at the input of said medium. l g

4. The system of claim 3 and in which the last mentioned means comprises means for making the rectied vvoltages impressed on` said grid. circuit substantially proportional 4to the square root of the signal'voltages received by saidfconf" trol circuit from said medium. j

5. The system of claim 8 andin which the 'last mentioned means'comprises means for making the voltages impressed on said rectifying means sub'- stantially proportional'to the square root oftlie' alternating current signal voltages impressed-'on'- said controlcircuit, andmeans for eifectively sup- Vpressing' from the rectied voltages before im'- pressing thein onv said grid circuit, YVa portion of 'I said frequency components. Y

6. In a signal transmission system, a source of 1 alternating current signals comprising frequena three-electrode space discharge devicehaving a grid circuitand an output circuit, said output circuit being connected effectively Vin shunt with said 105? medium so -as to insert an electrical lossA therein output circuit, a control circuit connected to the Vdependent on the value of theimpedance of saidv input of said medium soV as to receive aportioniof rectifier for rectifying said square root voltages, a

10W lpasslter for effectively suppressing from the 1 Yrectied voltages certain yaudio frequency com-` ponents, and means for impressing the filtered voltages on the grid circuit of Vsaid device in such sense as to makethe value of the' impedance of* said outputcircuit decrease withincrease in the 120 amplitude of the `signals at the input of said'- medium. i

7. In a signal transmission system','asource of alternatingcurrent signals within'the speech and musicfrequencyvrange, a transmission medium -1 upon which said signals are impressed' a pair of three-electrode space discharge devices having grid circuitsy and plate-lament circuits, said plate-filament circuits being4` connected in push`- pull relation with each othe'rand inbridgevvith 1 saidV medium so that an electrical loss,` the value of which is dependent on the value ofthe sum ofv the: impedances of said plate-filament circuits-is elfectivelyY inserted in said medium, means for picking oi a'portion of the vsignals from the input13`5` of said medium and forproducing therefrom volt-V ages which are substantiallyH proportional to the` square root of the picked-oir voltages, means forlinearlyrectifying said square root voltages, a low` pass lter for suppressingv from the rectified voltlages certain audio frequency components, andv means to impress the resulting rectiiiedvoltages simultaneously upcn the grid circuits cf :said devices in such sense as `to make the value of the impedances ofk their plate-filamentl circuits de crease with increase inthe amplitude of the fim` pressed rectifiedvoltages. I a v V8'. In combination in asignal transmission sys-` tem, la sourceof alternatingv current signals, a

transmission circuit suppliedswith said signalsya,

three-electrodespace discharge device having a grid circuit and a plate-filament circuit, said plate-filament circuit being Vconnected in shunt with a portion of said transmission circuit so as effectively to insert an electrical loss therein dependent on the value of the impedance of said plate-Filament circuit, and means to impress on said grid circuit, rectied voltages substantially proportional to the square root of the alternating current signal voltages supplied to the input of said medium, and in such manner as to make the value of the impedance of the plate-larnent circuit of said device decrease with increase in the signal amplitudes at the input of said transmission circuit, said portion of said transmission circuit being designed to have an impedance the value of Which is large in comparison with all values of the impedance of the plate-filament rcircuit of said 9. In combination in a signal transmission system, a source of alternating current signals, a long transmission circuit supplied with said signals, a receiving circuit connected to the output of said transmission circuit, a three-electrode space discharge device having a plate-filament circuit and a grid circuit,said plate-filament cir- Y cuit being connected effectively in series with a portion of said receiving circuit so as to insert an Vs0 '7 7 electrical loss in said receiving circuit the vvalue of amplitude of the signals at the input of said receiving circuit, said portion of said receiving circuitrbeing designed to have an impedance Which the plate-filament circuitof said device.

'NATHAN'IEL C. NORMAN.'

is low compared to all values of the impedance of 

